1. Field of the Invention
This invention relates to a negative-chargeability control resin for electrophotographic charging toners used when electrostatic latent images are made into visible images in dry-process electrophotography. More particularly, it relates to a negative-chargeability control resin which enables free control of electrostatic charging, may cause no toner scatter when used as a charge control agent and can contribute to formation of images having sharp hues, superior image reproducibility and high density.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 and Japanese Patent Publication No. 43-24748 are conventionally known as methods for electrophotography. As developing systems applied in these electrophotographic processes, they are roughly classified into dry-process developing systems and wet-process developing systems. The former is further classified into a system making use of two-component developers and a system making use of one-component developers.
As toners used in such dry-process developing systems, fine powders of natural or synthetic resins in which dyes or pigments have been dispersed are conventionally used. For example, particles of a binder resin such as polystyrene in which a pigment has been dispersed and which has been pulverized to have particle diameters of from 1 xcexcm to 30 xcexcm are used as a toner. As magnetic toners, those containing magnetic particles such as magnetite are used. In the case of the two-component developers, toners are also usually used in the form of a blend with carrier particles such as iron powder or magnetic ferrite particles.
Toners must electrostatically be charged positively or negatively in accordance with the polarity of the electrostatically charged image to be developed, and charge control agents are added in order to endow the toners with the desired triboelectric chargeability.
As charge control agents for positive triboelectric chargeability, nigrosine dyes, azine dyes, copper phthalocyanine pigments and quaternary ammonium salts or polymers having quaternary ammonium salts are known in the art. As charge control agents for negative triboelectric chargeability, metal complex salts of monoazo dyes and metal complex salts of salicylic acid, naphthoic acid or dicarboxylic acid are known in the art. However, the charge control agents are colored in many cases, and may cause a problem in reproducibility when used in color toners. Thus, chargeability control resins which are almost colorless or not so colored attract notice.
For example, Japanese Patent Application Laid-open No. 63-184762 (Japanese Patent Publication No. 8-12467) discloses a toner which has a binder resin comprised of a copolymer of styrene and/or xcex1-methylstyrene with an acrylic or methacrylic alkyl ester and contains a copolymer of styrene and/or xcex1-methylstyrene with 2-acrylamido-2-methylpropanesulfonic acid.
Japanese Patent Application Laid-open No. 2-167565 (Japanese Patent No. 2623684), Japanese Patent Application Laid-open No. 4-195166 (Japanese Patent No. 2609358), Japanese Patent Application Laid-open No. 7-72658 and Japanese Patent Application Laid-open No. 2000-305318 also disclose chargeability control resins which contain as constituent units acrylamide monomers containing a sulfonic acid group and toners which contain such chargeability control resins.
Japanese Patent Application Laid-open No. 8-123096 discloses a toner having i) a charge control agent containing an iron element and ii) a copolymer of a styrene/acrylic monomer with a sulfonic-acid-containing acrylamide monomer.
However, where such a copolymer of an acrylamide monomer having a sulfonic acid group with any other vinyl monomer is used as the chargeability control resin, the copolymer tends to have non-uniform monomer composition, making it difficult in some cases to endow the toner with stable triboelectric chargeability.
Japanese Patent Application Laid-open No. 9-166887 and Japanese Patent Application Laid-open No. 2001-305793 disclose toners containing a copolymer with a styrene/acrylic monomer having a specific carboxylic acid monomer (succinic-acid monohydroxyethyl methacrylate).
However, as described in these publications, the use of the above copolymer has made it necessary to use a conventionally known charge control agent in combination from the viewpoint of charge-providing performance.
Japanese Patent Application Laid-open No. 61-198249 discloses a toner comprised of a binder resin having as a constituent unit a monomer containing an amino group. This binder resin, however, is a positive-chargeability control resin containing an alkylene group in the monomer, and can not attain any satisfactory charging performance when applied in negatively chargeable toners.
The present invention is to provide a negative-chargeability control resin having solved the above problems. Accordingly, an object of the present invention is to provide a negative-chargeability control resin improved in uniformity of the copolymer composition constituting the negative-chargeability control resin and can endow the toner with stable charging performance.
That is, the present invention provides a negative-chargeability control resin comprising a vinyl copolymer obtained by copolymerizing a monomer composition which contains at least a vinyl monomer represented by the following formula (1) or (2) and a vinyl monomer having as a functional group a carboxyl group or a carboxyl group made into salt structure: 
wherein R1 represents a hydrogen atom or a methyl group; and R2 and R3 each represent a hydrogen atom or an alkyl group having 1 to 20 carbon atoms which can be substituted with a substituent except for an acid group, or R2 and R3 may combine chemically to form a heterocyclic ring having 4 to 20 carbon atoms, or R2 and R3 may form a cyclic structure having at least one atom selected from the group consisting of a nitrogen atom, an oxygen atom and a sulfur atom and having 4 to 19 carbon atoms; or 
wherein R4 represents an alkyl group having 1 to 20 carbon atoms, or an aromatic group.